timeout does not terminate the test process when used with xdist

Summary

running pytest-timeout with pytest-xdist doesn't terminate the test process.

This creates an issue if I would like to test a long-running computation-intensive function: the test process keeps running even after the timeout and the resources are not released until I manually kill the process.

Some version info from pytest

platform darwin -- Python 3.7.6, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/test_hang
plugins: cov-2.10.0, xdist-1.34.0, timeout-1.4.2, forked-1.3.0
timeout: 10.0s
timeout method: thread
timeout func_only: False

To reproduce

Create a simple test case (test_simple_hang.py) like this:

import time
import os
def test():
    print(os.getpid())
    time.sleep(60)

Running the following command:

pytest --timeout=10 --boxed --timeout_method=thread test_simple_hang.py

After the timeout, if I do ps -ef | grep <pid>, the process remains active.

It also occurs when --timeout_method=signal.

Start with the reproduced test in test_simple_hang.py and run the documented pytest command with both thread and signal timeout methods. Inspect how the timeout behaves with --boxed and xdist; done means the timed-out test process terminates and releases its resources instead of remaining active.